Method of drilling glass

ABSTRACT

A method of drilling holes in glass wherein a drill is rotated at a speed between about 3,400 rpm and about 3,600 rpm and advanced into a glass work piece at a constant forward rate of speed.

United States Patent 1191 Kelly 1 Jan. 16, 1973 METHOD OF DRILLING GLASSR r nces ited [75] lnventor: Joseph B. Kelly, Crestllne, Ohio UNITEDSTATES PATENIS [73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.3,568,367 3/197! Myers ..408/37 X I 2,941,338 6/1960 Santschi ..I25/20 XFlledr 19, 1971 2,320,874 6/1943 Lehmann... ....173/1sx x 3,401 ,5839/1968 Jacobson 1 ..408/1 30 [21] 116,970 2,322,237 6/1943 Johansen...402 137 x 3,124,016 3/1964 Reaser ..408/37 Related Dam 1,109,516 9/1914Dalton ..408/92 [63] Continuation-impart of Ser. No. 746,095, July 19,

I968, abandoned. Primary ExaminerDonald G. Kelly Attorney-Chisholm &Spencer [52] US. Cl ..51/283 511 1111.0. .3241; 1/00 [57] ABSTRACT [58]Field of Search...51/283, 81 R; 408/37, 92, 135, A method of drillingholes in glass wherein 11 drill is rotated at a speed between about3,400 rpm and about 3,600 rpm and advanced into a glass work piece at aconstant forward rate of speed.

6 Claims, 3 Drawing Figures SHEET 1 UF 2 J mdz y .5 J wk q w 120. A H\O& h fi w Q E 3 Mn METHOD OF DRILLING GLASS CROSS REFERENCE TO RELATEDAPPLICATION This application is a continuation-in-part of applicationSer. No. 746,095, filed July 19, 1968, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a method of drilling holes in hard refractories such asglass, concrete, and the like.

2. Description of the Prior Art Diamond drilling of holes in automotiveflat glass started in the mid-1950's with single holes in tempered ventwindows. At present, there are as many as holes in tempered side windows(sidelights). These holes benefit the automobile manufacturer in thatsavings result from simplified mounting arrangements through the holesand from the use of the tempered glass plate as a structural member tosupport its own lifting devices. In addition, holes in sidelights haveeliminated the use of a number of metal components formerly required inconjunction with the lifting devices.

As the volume of holes drilled grew from less than 2 million in 1956 to14 million in 1966 and over 18 million in l968, the manufacturingproblems associated with diamond drilling grew accordingly. Theseinclude excessive drill down-time, glass breakage, damaged drill bits,weak holes, and even closer center-to-center distances.

The general technique of drilling involves clamping the glass againstthe surface mounted between two diametrically opposed drill spindles.Diamond core drills are attached to the spindles, and cooling water issupplied to the center of the core by a revolving water joint. Drillingis accomplished by advancing a first drill about halfway through theglass and then retracting it; a

second drill is then advanced to remove the core and.

finish the hole. It is not practical to drill glass from only one side,because the surface opposite the drill will spall just prior tobreakthrough.

It has been customary to place holes in glass by applying a constantpressure or constant force to the drill. Constant-pressure drilling,with low drilling pressures, was chosen by glass producers because ofthe prevalent assumption that glass is weak and subject to breakage iftoo much force is applied against it. The idea was to keep the forceslow and let the drills ability to cut, i.e., the sharpness, determinethe cutting rate. When drilling time became too long, the drill wasresharpened by dressing it with an abrasive stone. Ideally, glass wouldnever be broken, regardless of the sharpness of the drill, becauseinsufficient force was available to break the glass.

With constant-pressure drilling, the time required to drill a hole is afunction of the sharpness of the drill and of the forces applied. Drillsharpness is always changing and is a difficult property .to control.The low drill forces necessary to prevent breakage are significantlyaltered by changes in friction in the system and therefore alsodifficult to control. The result is an unstable operation with drillingtime continuously varying from 6 to seconds. Sharpening is carried outabout every 300 holes to maintain output rates. Multiple drilling unitsrequire interlocks to insure that all drills have completed their cyclebefore a plate is removed. For

unknown reasons, some drill bits cannot be operated satisfactorily andhave to be discarded. Drill life averages 6,000 to 8,000 holes per pairof bits.

With constant-pressure drilling, the rate of advance of the drill bit isa function of the friction in the drilling apparatus, the sharpness ofthe drill, and the resistive forces of the glass. Each of these variesthroughout the life of the drill bit. Therefore, in constant-pressuredrilling, the rate of advance of the drill bit varies throughout thelife of the bit.

The most pertinent prior art of which I am aware comprises Lehmann, U.S.Pat. No. 2,320,874; Johansen, U.S. Pat. No. 2,322,237; and Santschi,U.S. Pat. No. 2,941,338.

The Lehmann patent relates, for example, to mining machines andparticularly relates to improvements in rotary drills for forming blastholes. Lehmann states that, A further object of the invention is toprovide a drill of the type mentioned which may be operated so that therate of infeed of the drill is adjustable and may be set so that therate of infeed is constant regardless of the resistance on the drillbit, or may be set so that the rate of infeed automatically varies indirect response to the amount of resistance to infeed of the materialbeing worked upon so that if less resistance is encountered, the rate ofinfeed increases, and if greater resitance is encountered, the rate ofinfeed decreases." Lehmann further states vulcanized rubber 32 isdisposed between the facing surfaces of members 24 and 28 and isvulcanized to such surfaces so as to provide a cushion for the spindlenose.

The Johansen patent relates, for example, to drilling apparatus, andmore particularly to improvements in a drilling apparatus of the highspeed, rotary, diamond core type. The reference states the combinedaction of the resistance to penetration of the work by the drill bit andthe resistance to rotation of the drill bit automatically effectscontrol of the feeding pressure of the bit.

The Santschi patent discloses, for example, an apparatus for drilling ahole in a glass sheet. The reference further states, Air under pressureby pipes 121 to air feed drills 30 and the exhaust of air from the aircylinders and drills 30 via pipes 120 and 146 start drills 30 so thatthrough the rotary-vane type of motor portion of each, spindles 50 andbit portions 62 and 63 are rotated. At the same time the air cylinderportion of each drill 30 moves the diamond-containing bit portions 62and 63 towards glass sheet G mounted between backup plate and clamp pad1 10.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod of drilling glass by rotating a drill at a speed suitable fordrilling glass, and advancing the drill into said glass at a constantrate of speed.

As used in this disclosure and claims, the terms constant feed,"constant rate," and constant feed rate, define a situation where therate of advance of the drill bit is independent of any resistive forcewhich the work piece may offer. Therefore, if the drill is set toadvance at a speed of 0.060 inches per second, the drill will advance atthis speed whether the drill is passing through air or through the workpiece. In this sense, the rate of advance of the drill is constant. Itis not intended, however, that the above-mentioned terms limit theinvention to a situation where the drilling operation is conducted at asingle speed. In some situations it may be desirable to vary the rate ofadvance of the drill. For example, while 0.060 inches per second is adesirable speed for drilling glass, there is no reason why the drillcannot be advanced at a rate of about 4 inches per second before thedrill contacts the glass. Additionally, with some refractories, it maybe desirable to vary the rate of advance of the drill as it penetratesthe refractory. This is still constant, as used in this disclosure, aslong as the rate of advance is altered by an external source, and not bythe resistance of the work piece. This is contrary to constant-pressuredrilling where the rate of advance of the drill is inverselyproportional to the resistance of the work piece.

In advancing the drill at a constant speed, I use a force far greaterthan that which is necessary, but I check this force so that only thepressure needed to keep the bit advancing at a given rate of speed isapplied to the drill. ln constant-pressure drilling, it is customary touse a force of about 50 to 60 pounds in drilling a /5 inch hole. Inconstant-feed drilling, I use an air cylinder that supplies about 300pounds of force, but a system is used to check the force, so that onlythe force necessary to move the drill at a constant rate is applied. Asthe resistive load changes, the force applied to the drill will changeto keep the rate of advance of the drill constant.

As far as l am aware, no one has ever considered constant-feed drillingas a solution to the problems of constant-pressure drilling in the glassindustry. This is because those skilled in the art felt that as the bitadvances, at a predetermined constant rate, it will produce a force thatis capable of breaking the glass. Applicant has discovered that this isnot true. Actually, this potential force is prevented from coming intocontact with the glass because an apparatus checks the potential force,so that only the force necessary to drill a hole at a given rate ofadvance reaches the glass.

ln constant-feed drilling, the depth of penetration of the drill bitinto the glass is determined by the rate of advance of the bit, thespeed of rotation of the bit, and the grit size of the diamonds. Unlikeconstant-pressure drilling, in constant-feed drilling, the depth ofpenetration is independent of variables such as friction in the drillingapparatus, sharpness of the drill bit, and resistance of the glass.Depth of penetration can there fore be controlled.

It has been customary in glass drilling to advance a drill bit at aconstant pressure. Theoretically, if the resistive forces were constant,it would be possible to choose a constant pressure that would yield aconstant rate of advance but, practically, it is impossible to determinesuch a constant pressure. In drilling a series of holes, conditions suchas friction due to drill sharpness and friction in the drillingapparatus change. With the resistive forces changing, there is no basisfor a constant force or pressure to move the drill at a constant rate ofadvance.

ln drilling any one hole, the resistive forces are more uniform, butthey are unpredictable. By this I mean that the resistive forces on thedrill bit do not vary a great deal during the drilling of any one hole,but it is impossible to determine, in advance, what the resistive forceswill be. For example, the frictional force caused by the drillcontacting the glass will not vary greatly when drilling a single hole,but there are several unknowns to be considered in computing this force.These unknowns are such that the force changes randomly from one hole tothe next, and so even though the force is relatively constant during thedrilling of a single hole, there is no way to ascertain what it will be.Since we cannot determine the resistive forces in advance, for even asingle hole, there is no basis for choosing a constant pressure thatwill yield a constant rate of advance.

It is not practical to choose a pressure that yields a decelerating rateof advance, since the speed will approach zero as the drill wears. (Dulldrills are slower than fast drills in constant-pressure drilling.) Sinceit is impossible to find a pressure that would yield a constant speed,and since deceleration is undesirable, the drill bit generallyaccelerates as it travels through the glass, in constant-pressuredrilling. This too is undesirable because the diamond drill bitpenetrates too deeply into the glass too quickly, leaving the glasssurface too close to the bonding material of the drill bit. With theglass surface close to the bonding material, there is little space forcooling fluid to circulate adjacent the bonding material and thereforethe bonding material tends to deteriorate more rapidly. in addition, theglass fragments that are removed from the glass by the drill, mix withthe cooling water and form an abrasive which also helps to deterioratethe bonding material. Since there is less cooling water, the glassfragment mixture is more concentrated, therefore being more abrasive tothe bonding material of the drill bit.

Test results indicate that constant-rate drilling has many advantagesover constant-pressure drilling. In particular, it is significant tonote that a drill bit has a useful life of from 10 to 25 times as longwhen it is advanced at a constant rate, rather than at a constantpressure.

A second major advantage of the constant-rate method is that drilldulling has no effect on rate of advance or depth of penetration of thedrill bit. In constant-pressure drilling, as the drill becomes dull, therate of advance of the drill decreases. When this rate becomes so lowthat it is impractical to continue drilling, the apparatus is shut downand the drill must be sharpened. In constant-feed drilling, the rate ofadvance is independent of the sharpness or dullness of the drill.Therefore, since dullness has no effect on the rate of advance, thedrill need not be sharpened as often. Further, where the drill isadvanced at a constant rate, time can be used to control all drillfunctions. No interlocks are required, and multiple drills can even becontrolled from one master timer.

Yet another advantage of constant-feed drilling is that sharpening canbe accomplished in a number of ways. Conventional stoning (drillingthrough a bonded stick of silicon carbide or aluminum oxide) works well,but requires positioning the stone and removing it after drilling, withresultant interruption of production.

The simplest and best method of drill sharpening, however, is merely toreverse the direction of drill rotation, then continue drilling. Byreversing the drills at predetermined times, it is possible tocompletely eliminate dulling problems. Using constant-pressure drilling,it does not appear to be possible to sharpen the drill successfullyafter the first reversal. This is probably due to the fact that inconstant-pressure drilling, the drilling takes place on the bondingmaterial at the base of the diamond, while with constant-feed drilling,the cutting takes place at the outermost diamond. Therefore, when onereverses the drill in the constant-feed method, the outermost diamond isremoved, this being the one that is dull. In constantpressure drilling,when one reverses the direction of rotation of a dull drill, the glassdoes not penetrate all the way to the bond material and therefore doesnot remove most of the dull diamonds at the outer portion of the drillbit.

It has been customary, in prior-art glass-drilling operations, to allowthe spindle to reciprocate and rotate within the motor housing. It isundesirable to allow the glass particles to get within the motor sincethey would destroy the motor. Therefore, a seal, commonly called aquill, has been used to keep glass particles from within the motor. Thishas not been very effective due to the fact that it is extremelydifficult to seal a shaft when there is both rotary and sliding motion.The present invention has solved this problem since there is noreciprocating motion of the spindle within the motor. Rather, the entiremotor reciprocates with the spindle. With this apparatus, it is nowpossible to effectively seal against ingress of the glass particlessince there is only rotary motion between the spindle and the motorhousing. There are a number of effective seals commercially available,but I have found that a simple labyrinth seal is most desirable.

DESCRIPTION OF THE DRAWINGS A complete understanding of the inventionmay be obtained from the foregoing and following description thereoftaken in conjunction with the appended drawings, which are diagrammaticand not to scale unless noted, and in which:

FIG. 1 is a diagram of a metal bonded diamond glass drill;

FIG. 2 is a schematic elevation view of the apparatus used in practicingthe method of the instant invention, i.e., apparatus for drilling glassby advancing the drill bit at a constant rate of speed; and

FIG. 3 is a schematic cross-section of a means for controlling the rateof advance of a drill.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isshown an enlarged view of a conventional metal bonded diamond glassdrill 10. Such drills have a glass abrading surface which comprisesdiamonds 14 bonded in suitable metals 16 (both of which have beenexaggerated in FIG. 1 for clarity). The glass abraiding surface of thedrill is annular in shape and drilling is accomplished by the abraidingforces created by the diamonds projecting from the drill. The drill ismounted on a hollow shaft 18 to provide a suitable rotary motion by anelectric motor.

Referring to FIG. 2, there is shown a drilling apparatus for carryingout the method of the present invention. The apparatus consists of fiveassemblies, namely: a support assembly 40, a drill assembly 60, an aircylinder 50, means 80 for controlling the rate of advance of a drill,and a work piece holder 110.

SUPPORT ASSEMBLY Support assembly 40 has a base plate 20 which extendsin a horizontal plane. Perpendicular shaft supports 22 are attached tobase plate 20 in a vertical position. While three shaft supports havebeen illustrated, it is to be understood that any suitable number may beused. FIG. 2 shows one of a pair of shafts 24 mounted on shaft supports22. The additional shaft 24 extends horizontally and is parallel to theone shown. As viewed in FIG. 2, the shaft 24 that is illustrated is infront of the one not shown. Both of these shafts are made of casehardened steel to avoid deflection. Base structure 41 is constructed ofheavy steel plates with reinforcing ribs 42. Cylinder mount 44 extendsin a vertical direction and is rigidly connected to base plate 20 at oneend thereof. Cylinder mount 44 consists of an elongated metal plate thatextends vertically beyond air cylinder 50.

DRILL ASSEMBLY Drill assembly 60 acts to rotate the drill from about3,400 rpm to about 3,600 rpm. The drill assembly includes an electricmotor 62 which rotates a spindle 63. At the forward end of motor 62 is arotary seal 61 to prevent ingress of abrasive glass particles into themotor. A simple labyrinth seal has been found to be very effective. Atone end of spindle 63 is a drill chuck assembly 64 for receiving hollowshaft 18. In the embodiment illustrated, the drill 10 is water cooled.Spindle 63, which is hollow, is supplied with water from its rear end byany suitable source, such as a hose 59 connected to spindle 63 by meansof a rotating water joint 61. Cooling water flows from spindle 63 toshaft 18 and then to drill 10. A petcock 65 can be used as a limitingorifice to prevent water from surging into the drill when the drill isnot engaging the glass work piece.

Drill assembly 60 has roller mounts 26, one pair mounted at each end ofthe drill assembly. A rolling V- bearing assembly is positioned on eachroller mount 26. Each roller mount 26 is designed to be almostfrictionfree. It consists of a circular plate 30 and a pair of rollers32. Each shaft 24 is disposed between a pair of rollers 32 and incontact with them. The circular plate 30 can be adjusted to regulate thepressure of the rollers 32 on the shaft 24. This pressure contact of therollers against the shaft insures that the drill assembly is movedtoward the glass by themovement of air cylinder 50.

AIR CYLINDER Drill assembly 60 is moved toward the glass work piece byan air cylinder 50 and an associated piston rod 52. Bracket 54 isrigidly connected to cylinder mount 44, and piston rod 52 is pivotally'connected to bracket 54 by a pin 56. Attached to the forward side of aircylinder 50 by a pin is a bracket 66. This bracket is attached to motor62 by bolts 68.

Using a diamond drill having a grit size of 6080 mesh, a forward oradvancing dlrill speed of 0.040 to 0.100 inches per second isacceptable, and a drill speed of 0.050 to 0.060 inches per second ispreferred. With a coarser grit size the advancing speed may beincreased. Speeds below 0.040 inches per second are not practical sincedrilling time increases and the life of the drill bit decreases.

MEANS FOR CONTROLLING THE RATE OF ADVANCE OF THE DRILL The apparatusheretofore described is capable of advancing a drill at a constantpressure into a glass work piece G. By adding a means for controllingthe rate of advance of the drill, I change from a constant-pressureoperation to a constant-speed operation. I do not claim to have inventeda means for controlling the rate of advance of a piston-cylinderarrangement and several such means are now commercially available. Onesuch means is a Bellows HYDRO-CHECK, sold by the Bellows-ValvairCorporation of California. FIG. 2 shows a HYDRO-CHECK 80 in position tocontrol the rate of advance of cylinder 50 (and therefore drillReferring to FIG. 3, there are shown the details of HYDRO-CHECK 80. Itincludes a piston rod 82 extending into a checking cylinder 86. As thepiston rod 82 is pulled out, a moving piston 81 forces oil in the frontportion 84 of the checking cylinder 86 through a transfer tube 88 and aneedle valve 90 into the rear portion 92 of checking cylinder 86. Therate of flow of the oil is determined by the setting of a knurled knob94 which controls the size of the passage through needle valve 90. Thus,the rate at which piston 81 (and therefore piston rod 82) can beadvanced may be controlled with accuracy. On the return stroke, one-wayvalve 96 permits oil to flow freely through the piston.

The spring-loaded balance cylinder 98 acts as a reservoir for the volumeof oil displaced by the piston rod 82 on the inward stroke, and returnsthis amount to the checking cylinder 86 on the outward stroke. Indicatorrod 100, attached to the balance cylinder piston, indicates the amountof oil in the HYDRO-CHECK and when oil should be added.

Brackets 102, 103 and 104 are used to rigidly mount HYDRO-CHECK 80 tocylinder mount 44.

At the forward end of the HYDRO-CHECK assembly is a bracket 105 which isrigidly connected to air cylinder 50 by a bolt 107. In the upper end ofbracket 105 is a hole 108 which is larger in diameter than the outsidediameter of piston rod 82 so that the rod 82 can slide freely thereon.Mounted on piston rod 82 are three nuts 109A, 109B, and 109C. Nut 10913is positioned so that as bracket 105 moves to the left due to theactuation of the air cylinder, bracket 105 engages nut 109B just priorto drill 10 contacting the glass work piece G. Any further movement tothe left of bracket 105 will be limited by the opening of the needlevalve 90 in HYDRO-CHECK 80, so that the advance of drill 10 is at aconstant rate when the drill penetrates the work piece G.

WORK PIECE HOLDER A work piece holder 110 is positioned adjacent thedrill assembly. A U-shaped base plate 111 provides a mounting for amovable back plate 112 and a fixed face plate 114. A connecting arm 118is pivotally attached to base plate 111 by any suitable means, such as apin 120. Arm 118 is pivotally connected to back plate 112 by pin 122 atone end and to piston rod 124 by pin 123 at the other end. Back plate112 clamps the work piece G against face plate 114 by the actuation ofthe piston rod 124 in a cylinder 126, which is attached to base plate20. Both the back plate 112 and the face plate 114 have openings thereinto permit passage ofa drill.

OPERATION The first step in the method is to adjust the opening inneedle valve by turning knurled knob 94. This sets the rate of advanceof the drill. A work piece G is then positioned against a face plate 114and a back plate 112 is brought into contact with the work piece G toclamp the work piece in place. Pressure in air cylinder 50 causes thecylinder and therefore the drill assembly 60 to move to the left.Cylinder 50 should be capable of producing at least a ZOO-pound thrustand a 300-pound thrust is preferred. The size of the drill varies,depending on the hole size desired, but I have used a drill having anoutside diameter of 0.523 inches and a wall thickness of 0.040 inches toproduce a hole about 0.525 inches in diameter. Drill 10 is subject tothe entire pressure within air cylinder 50 and the initial movement ofthe drill is therefore quite rapid. This is desirable since there is noneed, at this time, to advance the drill at a speed that is slow enoughto be suitable for drilling into glass. Nuts 109A and 109B are placed onpiston rod 82 in such a manner so that bracket engages nut 109B justbefore drill 10 engages the glass work piece G. In order for the bracket105 to continue its movement to the left, it must move piston rod 82. Tomove piston rod 82, oil must be forced through transfer tube 88 andneedle valve 90. Since the fluid in the HYDRO-CHECK is substantiallynon-compressible, the rate of movement of the piston rod 82 isdetermined solely by the opening in needle valve 90, which can be set byknurled knob 94.

When the drill 10 has penetrated a desired amount into the work piece G,a timer (not shown) returns the air cylinder and therefore the drillassembly to its original position. Note that as the air cylinder 50retracts to the right, bracket 105 engages nut 109C and therefore causespiston rod 82 to return to its original position. The apparatus is nowin position to commence drilling'another hole.

Although only one complete apparatus has been shown, it is normallydesirable, when drilling into glass, to drill from both ends, i.e.,advance the first drill 10 to approximately the middle of the workpiece, retract the first drill, and advance a second drill 10 from theopposite side of the work piece to remove the core and finish the hole.It is not practical to drill glass from only one side, because thesurface opposite the drill will spall just prior to breakthrough.

It has been customary in the glass-drilling art, when using two opposingdrills, to have the first drill apply its force against the movableplate. The theory behind this is that the glass is strongest before thefirst drill removes any glass. The second drill finds the glass workpiece weaker than the first drill found it. Since the glass is weaker,it offers less resistance to the drill and it needs more support toavoid any unwanted deflection. Ordinarily, the movable plate is weakerthan the fixed plate. Therefore, the movable plate is used to supportthe glass from behind during the first drilling operation.

I have found that unless the movable support is strengthened, so that itholds the work piece without letting it deflect, the constant-feeddrilling apparatus deteriorates to a constant-pressure apparatus. Thisis because the glass, without sufficient support, is moved by the drillfor a distance which is a function of the pressure applied to the glass.Therefore,-the relative speed of the drill with respect to the glass isno longer constant. Tests have indicated that a movable plate capable ofwithstanding 200 pounds of thrust without significant deflection isinadequate since it allows the glass to deflect. The movable supportshould be capable of supporting the glass without any glass deflection.If the support is capable of supporting 750 pounds of thrust, it isacceptable, but 1,000 pounds is preferred. It is helpful to make thehole in the movable plate very close in size to the outside diameter ofthe drill bit to provide maximum support.

I claim as my invention:

l. A method of drilling a glass work piece which comprises rotating adrill at a speed suitable for drilling glass, and advancing the drillinto said glass at a constant rate of speed, said drill being advancedby a force which is greater than that necessary to advance said drill,but which is checked so that only the force necessary to advance saiddrill at a given constant rate of speed is applied thereto.

2. In a method of drilling a glass work piece the method comprising thesteps of:

a. positioning a glass work piece adjacent a drill;

b. rotating said drill at a speed suitable for drilling glass; and

c. advancing said drill into the surface of said glass work piece at aconstant forward rate of speed, said drill being advanced by a forcewhich is greater than that which is necessary to advance said drill, butwhich is checked so that only the force necessary to advance said drillat said constant rate of speed is applied thereto.

3. A method of drilling glass wherein the drill is advanced into theglass at a constant rate of speed of about 0.055 inches per second whilethe drill is rotating at about between 3,400 revolutions per minute and3,600 revolutions per minute, said drill being advanced by a force whichis greater than that which is necessary to advance said drill, but whichis checked so that only the force necessary to advance said drill atsaid constant rate of speed of about 0.055 inches per second is appliedthereto.

4. A method of drilling a hole in a glass work piece which comprises:

a. rotating a first drill at a speed suitable for drilling a hole inglass;

b. rotating a second drill at a speed suitable for drilling a hole inglass; and

c. advancing said first and second drills into the glass work piece fromopposite sides at a constant rate of speed, each of said drillsadvancing at least to the approximate midpoint of said work piece, eachof said drills being advanced by a force which is greater than thatwhich is necessary to advance said drills but which is checked so thatonly the force necessary to advance said drills at said constant rate ofspeed is applied thereto.

5. A method of drilling a glass work piece comprising the steps of:

a. positioning a glass work piece adjacent a drill;

b. rotating said drill at a speed suitable for drilling glass; and

c. advancing said drill into a surface of said glass work piece at aforward rate of s e ed that is independent of the sharpness of the H1],the resistive forces of the glass, and the frictional forces in adrilling apparatus which is advancing said drill, said drill beingadvanced by a force which is greater than that which is necessary toadvance said drill, but which is checked so that only the forcenecessary to advance said drill at said con stant rate of speed isapplied thereto.

6. A method of drilling a glass work piece which comprises the steps of:

a. positioning a glass work piece adjacent a drill;

b. rotating said drill at a speed suitable for drilling glass; and

c. advancing said drill into a surface of said glass work piece in sucha manner that the depth of penetration of the drill bit into said glasswork piece is determined by the rate of advance of said drill bit, thespeed or rotation of said drill bit, and the grit size of diamonds insaid drill bit, and is independent of the sharpness of the drill bit,the resistance of the glass work piece, and the frictional forces in theapparatus advancing the drill bit, said drill being advanced by a forcewhich is greater than that which is necessary to advance said drill, butwhich is checked so that only the force necessary to advance said drillat. said constant rate of feed is applied thereto.

1. A method of drilling a glass work piece which comprises rotating adrill at a speed suitable for drilling glass, and advancing the drillinto said glass at a constant rate of speed, said drill being advancedby a force which is greater than that necessary to advance said drill,but which is checked so that only the force necessary to advance saiddrill at a given constant rate of speed is applied thereto.
 2. In amethod of drilling a glass work piece the method comprising the stepsof: a. positioning a glass work piece adjacent a drill; b. rotating saiddrill at a speed suitable for drilling glass; and c. advancing saiddrill into the surface of said glass work piece at a constant forwardrate of speed, said drill being advanced by a force which is greaterthan that which is necessary to advance said drill, but which is checkedso that only the force necessary to advance said drill at said constantrate of speed is applied thereto.
 3. A method of drilling glass whereinthe drill is advanced into the glass at a constant rate of speed ofabout 0.055 inches per second while the drill is rotating at aboutbetween 3,400 revolutions per minute and 3,600 revolutions per minute,said drill being advanced by a force which is greater than that which isnecessary to advance said drill, but which is checked so that only theforce necessary to advance said drill at said constant rate of speed ofabout 0.055 inches per second is applied thereto.
 4. A method ofdrilling a hole in a glass work piece which comprises: a. rotating afirst drill at a speed suitable for drilling a hole in glass; b.rotating a second drill at a speed suitable for drilling a hole inglass; and c. advancing said first and second drills into the glass workpiece from opposite sides at a constant rate of speed, each of saiddrills advancing at least to the approximate midpoint of said workpiece, each of said drills being advanced by a force which is greaterthan that which is necessary to advance said drills but which is checkedso that only the force necessary to advance said drills at said constantrate of speed is applied thereto.
 5. A method of drilling a glass workpiece comprising the steps of: a. positioning a glass work pieceadjacent a drill; b. rotating said drill at a speed suitable fordrilling glass; and c. advancing said drill into a surface of said glasswork piece at a forward rate of speed that is independent of thesharpness of the drill, the resistive forces of the glass, and thefrictional forces in a drilling apparatus which is advancing said drill,said drill being advanced by a force which is greater than that which isnecessary to advance said drill, but which is checked so that only theforce necessary to advance said drill at said constant rate of speed isapplied thereto.
 6. A method of drilling a glass work piece whichcomprises the steps of: a. positioning a glass work piece adjacent adrill; b. rotating said drill at a speed suitable for drilling glass;and c. advancing said drill into a surface of said glass work piece insuch a manner that the depth of penetration of the drill bit into saidglass work piece is determined by the rate of advance of said drill bit,the speed or roTation of said drill bit, and the grit size of diamondsin said drill bit, and is independent of the sharpness of the drill bit,the resistance of the glass work piece, and the frictional forces in theapparatus advancing the drill bit, said drill being advanced by a forcewhich is greater than that which is necessary to advance said drill, butwhich is checked so that only the force necessary to advance said drillat said constant rate of feed is applied thereto.